1. Field of the Invention
The present invention relates to a digital/analog (D/A) converter of high accuracy and high bit, which is used in, for example, a compact disc player, a sound synthesizer and the like.
2. Description of the Prior Art
The D/A converters of various systems have hitherto been put into practical use, which are roughly divided into an amplitude modulation (AM) type and a pulse width modulation (PWM) type.
The PWM type D/A converter is more advantageous than the AM type one in its simple construction of circuit and no need for an element of high accuracy (a resistance of high accuracy), while it is disadvantageous in slow conversion speed, or large harmonic distortion. Therefore, the inventors have proposed a D/A converter in which the PWM type and AM type D/A converter is combined (disclosed in the Japanese Patent Laid-Open No. 57-23321, 1982), which processes the digital data of the high order M bits among the input digital data of N bits by use of the AM type D/A converter which utilizes the resistance division, and the digital data of the low order (N-M) bits by use of the PWM type D/A converter which utilizes an output of a counter for counting clock pulses.
In such a hybrid type converter need not utilize resistance for the resistance division of high accuracy such as used in the AM type D/A converter which is given the entire N bits. In addition to the said advantage as in the PWM type A/D converter it also has another advantage that the conversion speed is fast as in the AM type one. However, the defect of large harmonic distortion has not been eliminated. For example, when the hydrid type D/A converter, which processes the high order 8 bits of the 16 bits digital data by an AM type D/A converter circuit and the low order 8 bits by a PWM type D/A converter circuit, is given the digital data of 16 bits corresponding to an analog value of 1 kHz and full-scale amplitude to thereby digital/analog-convert the data by a conversion frequency of 44.1 kHz, the harmonic distortion of the output analog signal at 20 kHz or less is about 0.05% in simulation by the fast Fourier transform (FFT). Such harmonic distortion can be said to be very large compared with the digital data of 16 bits which inherently has the resolution of 0.0015% (=100.div.2.sup.16).
Thus, the conventional D/A converter has the problem of the resistance of high accuracy being required, or the harmonic distortion being larger.